This disclosure relates to mechanical joints, particularly those used with sleeve-and-rack assemblies.
It is known to provide power assist to a steering assembly by providing a sleeve surrounding and threadingly engaged with the rack such that rotation of the sleeve by a motor imparts force to the rack, thereby assisting the driver in steering the vehicle. However, providing such a sleeve causes the steering system to be overconstrained, thereby resulting in high friction and excessive wear and tear. Such systems as have actually been put into production show high friction and wear at the sleeve, indicating that the rack is not moving linearly along the axis of the sleeve, thereby causing strain. This implies that prior art sleeve-and-rack systems are overconstrained. What is needed is a means to relieve the constraint.
Disclosed is a xe2x80x9cvirtual ball jointxe2x80x9d that comprises a pair of bearings, each having outer races with angled surfaces coinciding with a spherical surface, thereby allowing spherical motion of the inner and outer races relative to one another about a center of curvature. A shaft mounted in the bearings is thereby able to rotate about the center of curvature as though mounted with a ball joint. The device is particularly useful for mounting ball screw rack sleeves used in power assisted sleeve-and-rack steering assemblies. By so doing, two degrees of freedom are added to the power assist system, thereby transforming the sleeve-and-rack assembly from an overconstrained mechanical system to an underconstrained one. It is further disclosed that the load bearing capabilities of such a virtual ball joint are adequately preserved so long as the relative spherical motion of the bearing races is limited to no more than a fraction of a degree.